This invention concerns a method for preparing a thermoplastics tube manufactured by spirally winding and welding together of a hollow profile having a substantially rectangular cross section.
A drawback of spirally wound tubes of the above mentioned type is that on cutting the tube no unbroken cutting surface is received, but the cutting surface comprises only an outer and an inner edge as well as two short portions where the cut passes through radially directed side walls of the hollow profile. This results in that dirt easily will penetrate into the spirally extending cavity between the inner and outer wall of the tube and further, joining of these tubes will become difficult due to relatively thin and unevenly applied material thickness in a cut tube end.
Previously attempts have been done to overcome this, for instance according to EP-A 0 540 770, by turning a groove of uniform width and depth between the outer and the inner tube wall in an end of a spirally wound double wall tube, and then heating the plastics material in the tube walls defining the turned groove. A specially made mould sealing ring is possibly inserted in the turned groove, and then the heated tube end with or without said sealing ring is inserted into a conically tapering groove in an annular mould, whereby the tube end will get a wished shape. If no sealing ring is used the tube end must, after cooling, still be provided with a welding seam closing a narrow gap between the outer and inner tube walls, which gap will remain when the tube end has been moulded. This previously known method is primarily intended to mould and prepare a male and a female end of a socket pipe. A drawback of the method is that it requires a lot of extra measures in the tube end and comprises a lot of time consuming working steps as well as specially made accessories.
A considerably simplified method is described in the Finnish patent application No. 964518, in which a cut tube end is heated until the plastics material is changed into a plastic condition, and then the tube end is inserted into an annular groove in a press mould corresponding to the cross section of the tube and is exerted to an axially directed pushing force. The plastics material in the end of the tube is hereby pressed together so that a continuous planar end face is achieved. According to another embodiment a string of molten plastics mass is extruded onto the bottom of the annular groove before the tube end is inserted. This method is relatively suitable for spirally wound tubes having small diameters, but when the dimensions of the tubes increase this method will be too clumsy.
The object of the present invention is to provide a method which is as suitable for large as for small spirally wound tubes, and which will result in tube ends which may be joined together according to same methods as solid tubes. This is achieved by a method, which is characterized by the steps as follow:
cutting the end of the tube by a planar transverse section;
extruding a second hollow profile having a rectangular cross section and an outer shape essentially similar to that of the hollow profile from which the tube is manufactured;
spirally winding the extruded second hollow profile around a drum having a diameter corresponding to inner diameter of the tube;
cutting a loop from the second hollow profile in a length corresponding to circumference of the tube end;
joining the ends of the loop to each other, whereby a ring is achieved having same inner and outer diameters as the spirally wound tube;
placing the ring against the cut tube end, and
welding the ring to the tube end.
The second hollow profile is preferably manufactured with a somewhat larger wall thickness than the hollow profile of the tube itself. In tubes produced from small profiles the second profile is therefore suitably extruded as a solid profile without cavity.
Further features of the invention will appear from the dependent claims 3 to 6.